Tesco has come under increasing pressure from customers to stop participating in government unemployment schemes which allow the company to take on jobseekers to stack and clean shelves for up to eight weeks without paying them.

After a link to a job centre advert was posted on Twitter on Wednesday evening, appearing to show that the supermarket giant was hiring for a permanent role as a night shift worker paying only jobseeker's allowance, customers began bombarding the supermarket chain with complaints on Twitter and on the company's Facebook site and threatening to withdraw their custom.

Tesco said an IT error was behind the posting. Under current government rules it is illegal to take on jobseekers for longer than six months without paying them. Tesco confirmed it does participate in the Department of Work and Pensions Work Experience scheme in which people claiming jobseeker's benefit can be asked to work in charities and major profitable high street chains for up to eight weeks at 30 hours a week. If claimants withdraw from the scheme after a week they can have their benefits stopped.

The Guardian has uncovered other adverts for similar unpaid Tesco roles posted this month in Clevedon and in Dinnington. Britain's largest private employer, which made over £3.5bn in profit last April, said that it had taken on 1,400 such claimants in the last four months. This amounts to 168,000 hours of unpaid work if all participants in the scheme work for 30 hours a week.

The campaign group Boycott Workfare has said it is organising a protest for 3 March to target firms involved in what it has described as modern-day slave labour.

Tesco said: "The advert is a mistake caused by an IT error by Jobcentre Plus and is being rectified. It is an advert for work experience with a guaranteed job interview at the end of it as part of a government-led work experience scheme."

Tesco's competitor Sainsbury's recently informed the Guardian that it had withdrawn from all government schemes which were not entirely voluntary and threatened receipt of benefits in any way.

Tesco also confirmed that it did not take on anyone under the Mandatory Work Activity scheme which lasts for four weeks, but which, if referred, jobseekers must take part in or lose benefits for three months.

A Tesco spokesperson said: "We take our responsibility as Britain's biggest private sector employer seriously and are playing our part to help tackle unemployment in these challenging times.

"We are participating in a government-led voluntary work experience scheme to help give young people valuable experience of the workplace. Over 300 young people have recently gained a paid job at Tesco following their work experience in recent months."